In the 16th century, Europe experienced a long series of nasty and violent religious wars. With Christianity splitting into many sects, each one wanted its own political power. Once a sect gained that power, it used it to oppress the others. The oppressed sects then fought that much harder to achieve their own independence.
Into this fray of religious warfare, Thomas Hobbes entered and proposed a solution: Instead of fighting about which religion would hold sovereign power so as to extend its influence, we could all just collectively decide that sovereign power would only promote peace and stability for its citizens. By defining sovereignty down, Hobbes hoped to avoid bloody religious warfare. Amidst this redefined sovereignty, Hobbes proposed picking one overriding religion—it didn’t really matter which one since all were equally untrue—and imposing it on all.